Particulate trap for purifying diesel engine exhaust

ABSTRACT

In combination with a diesel engine, a particulate trap for purifying the diesel exhaust of the engine includes a case provided in the exhaust line of the diesel engine and a filter element mounted in the case. The filter element is a porous member of a heat-resistant metal framework having the form of a three-dimensional mesh. The surface roughness of the metal framework is at least 0.2 μm. The pores of the porous member have an average diameter of 0.1-1 mm, the average number of such pores as counted in the thickness direction of the porous member is 10 or more, the volume packing density of the filter element to the entire filtering space is 10-40%, and the filter has a gas inflow filtration area of 400 cm 2  per liter of the displacement of the engine to which the trap is to be mounted. The filter element has a high capacity to collect particulates and can withstand thermal shock during regeneration and yet produces little pressure drop in the exhaust line during filtration.

BACKGROUND OF THE INVENTION

This invention relates to a particulate trap for collecting and removingparticulates such as carbon contained in exhaust gas discharged from adiesel engine.

Exhaust gas discharged from car engine is one of the major causes of airpollution. It is therefore important to develop techniques for removingharmful components contained in exhaust gas.

It is especially important to remove particulates contained in theexhaust gas discharged from diesel engines, which are mainly NOx andcarbon.

Heretofore, various efforts have been made to remove such harmfulcomponents before the exhaust is discharged from the engine. Suchefforts include putting EGR (exhaust gas recirculation) on the engineand improving a fuel injector and the shape of a combustion chamber.However none of these measures is a definitive solution. Another knownmeasure is to provide an exhaust trap in the exhaust passage to collectthe particulates contained in the exhaust (as proposed in UnexaminedJapanese Patent Publication 58-51235). This method in which the exhaustis treated after being discharged from the engine, is considered morepractical and has been studied vigorously.

Such a particulate trap for collecting particulates in exhaust gas hasto satisfy the following requirements in view of the conditions of use.

First, it has to be capable of collecting particulates with such a highefficiency that the exhaust is cleaned sufficiently. Each country setsby law a different upper limit of the particulate emissions. Forexample, the Japanese government has set a long-term target of upperlimit at 0.08 g/Km in the 10-mode test in passenger cars, light trucksand buses, which is to be attained by the year 2000. In the U.S., the1994 EPA restriction has set such an upper limit at 0.1 g/HP.Hr forheavy-duty cars in the transient mode and at 0.08 g/mile for light-dutycars in the LA-4 mode. These are pretty tough regulations. The amount ofparticulates contained in the exhaust depends on the displacement of thediesel engine and the load applied. However it is generally consideredthat such a trap has to be capable of collecting, on the average, 60% ormore of the particulates in the exhaust to meet such regulations.

Secondly, the pressure drop of exhaust gas must not be so large. As theexhaust gas passes through the trap and the particulates therein arecollected by the trap, its resistance to gas flow increases. If theresistance is too great, an undesirable back pressure will act on theengine. It is therefore necessary to restrict such back pressure below30 KPa after collecting particulates. For this purpose, it is necessarythat not only the initial pressure drop be sufficiently low, but theresistance to the flow of exhaust be small, i.e. the pressure drop doesnot rise too much even after the particulates have been collected.

Furthermore, it is necessary to periodically remove the collectedparticulates when a predetermined amount of particulates has beencollected to regenerate the trap so that it can regain its initialparticulate collection capacity. Thus, the third requirement for aparticulate trap is that it is tough enough to endure repeatedregeneration treatments. It is considered the most promisingregeneration method to burn particulates with an electric heater or aburner. In either case, the collected particulates have to be heated toa temperature higher than the ignition point of the particulates(usually 600° C.). Recycling treatment, i.e. the burning of thecollected particulates, has to be completed before the back pressureincreases to such a degree that the engine performance drops or thedrivability worsens. After regeneration, particulates are collectedagain. Trappings and regenerations are repeated. Thus, the pressure dropis always kept at a level below a predetermined value. It is thereforeimportant that the filter element be made of a material which can notonly sufficiently withstand the repeated regeneration treatments butshow sufficiently high resistance to corrosion by the atmospheric gascontained in the exhaust.

The above requirements are met if a honeycomb-shaped porous member madeof cordierite ceramic is used as the filter member in the particulatetrap. Such a filter is also considered most practical. However with thisconventional arrangement, in which the particulates collected by thecordierite ceramic are burned, the filter is repeatedly heated to arather high temperature. Thus, the filter frequently melted or developedcracks due to thermal shock when the filter was heated for regenerationand then cooled down.

Because this type of filter element has a problem in that theregeneration control is extremely difficult, it is not practically usedyet.

SUMMARY OF THE INVENTION

The present invention was made to solve the above-mentioned problems andits object is to provide a particulate trap for purifying diesel engineexhaust which can collect particulates with high efficiency while notincreasing the pressure drop and which can withstand thermal shocks whenit is heated and cooled for regeneration.

For this purpose, it is necessary that the pores of the filter elementsfor collecting particulates have a suitable diameter and that thefiltering parts of the filter elements such as their fibers andframework be thick enough so that they can collect particulates easily.The particulate collection capacity is also influenced by the size andsurface condition of the particulate collection portions of the filterelements.

In order to satisfy both of the mutually conflicting requirements forhigher particulate collection capacity and for lower back pressure, itis necessary to design a trap so as to have a fairly large filtrationarea at the exhaust inflow side while keeping the entire trap compact.

In order to attain the above object, the present inventors found outthat a trap having three-dimensional mesh-like porous filter elementsmade of a heat-resistant metal and having communicating pores has a highparticulate collection capacity, is less likely to increase pressuredrop and is difficult to melt or crack due to temperature increase whenthe collected particulates are burned for regeneration.

The metallic three-dimensional mesh-like porous material may be, e.g. aporous metal (made by Sumitomo Electric Industries under the name of"Celmet") produced by subjecting a foamed urethane having communicatingpores and having a three-dimensional mesh-like structure toconductivity-imparting treatment and electroplating it.

As shown in FIG. 1, the three-dimensional mesh-like porous member is aporous framework or skeleton 3 having pocket-like pores 2. Since it hasa high porosity, once particulates are collected in the pores, theycannot easily escape despite the fact that the resistance to the gasflow is relatively low. In other words, it has a high particulatecollection capacity.

The present inventors also tried to determine the proper ranges of thepore diameter of the three-dimensional mesh-like porous member havingcommunicating pores, the number of pores as counted in the thicknessdirection of the filter element, and the volume packing density at whichthe metal framework or skeleton occupies to the entire filtering space,and the relation between the exhaust inflow filtration area of thefilter element per liter of the displacement of the engine to which thetrap is mounted and the particulate collection capacity and the pressuredrop.

The relation between the pore diameter of the communicating pores formedin the three-dimensional mesh-like porous member and the filterperformance will first be described. The pore diameter of thethree-dimensional mesh-like porous member has to be determined so as toprovide for both high particulate collection capacity and low pressuredrop in a balanced manner. The term "pore diameter" used herein refersto the diameter of the pores formed in the porous framework andcorresponds to the diameter of the bubbles formed when thethree-dimensional mesh-like porous member is formed by foaming. It wasfound out that the average pore diameter should be 0.1-1 mm forcollection efficiency. If the average pore diameter is less than 0.1 mm,though the particulate collection efficiency improves, the resistance toair-flow will increase quickly to such an extent that the back pressureon the engine exceeds 30 KPa, which puts too heavy a load on the engine.If the average pore diameter is more than 1 mm, an increased amount ofthe particulates will pass through the filter element without gettingcollected, especially while the filter element is new or immediatelyafter the filter has been regenerated. Thus, the particulate collectionefficiency will drop below 60%.

The present inventors also examined the preferable range of the numberof pores in the thickness direction when using the porous material as afilter. If the average number of pores having diameters of 0.1-1 mm ascounted in the thickness direction of the filter element (including anypore crossed even the least bit by a line extending in the thicknessdirection) is 10 or less, the collection capacity tends to be too low.

If the volume packing density of the metal skeleton of thethree-dimensional porous filter element to the entire filtering portionis less than 10%, the particulates are less likely to collide with andattach to the framework. The particulate collection capacity is alsolow. If the ratio is more than 40%, though the collection capacity ishigh, the pressure drop tends to increase to an unacceptably high level.Thus, the ratio is preferably not more than 40%.

If the exhaust inflow filtration area of the filter element per liter ofthe displacement of the engine to which the trap is to be mounted is 400cm² or less, this means that the inlet of the filter element throughwhich the particulates pass is too small. Thus, the exhaust tends toflow at a rather high speed through the filtering portion, therebyunduly increasing the pressure drop.

Further, the present inventors examined in detail the relation betweenthe projected width of section of the metal framework making up thethree-dimensional mesh-like porous member (projected length of a side ofa section of the metal framework) and the filter performance. As aresult, it was found out that the filter performance is good if theprojected width is 20 μm or more. If less than 20 μm, an increased partof the particulates tend to pass through the metal framework withoutcolliding with it, resulting in reduced particulate collection capacity.Thus, the projected width of the framework should preferably be not lessthan 20 μm.

The present inventors also examined the relation between the surfaceroughness Rmax of the metal framework forming the three-dimensionalmesh-like porous member and the particulate collection capacity and thepressure drop. As a result, it was found out that, by using athree-dimensional mesh-like porous member having a surface roughnessRmax of 0.2 μm or more, the filter performance improved sufficiently. Ifthe surface roughness Rmax is less than 0.2 μm, the particulates oncecollected tend to be blown away by the subsequent exhaust gas flow,resulting in reduced particulate collection capacity as a whole. Thegreater the Rmax value, the higher the initial particulate collectioncapacity tends to be. However as the amount of particulates collectedincreases, the surface condition of the framework has a smallerinfluence on the particulate collection capacity. By controlling thesurface roughness of the framework, it is possible to improve theinitial particulate collection capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged view of a three-dimensional mesh-like porousmember having communicating pores.

FIG. 2 is a perspective view of a filter element formed by spirallywinding a three-dimensional mesh-like porous sheet.

FIG. 3 is a perspective view of another filter element formed byconcentrically arranging a plurality of three-dimensional mesh-likeporous sheets one on another.

FIGS. 4(A)-9(A) are vertical sectional views of various embodiments ofthe particulate trap.

FIG. 4(B)-9(B) are cross sectional views thereof.

FIG. 10 is a perspective view of an embodiment of the particulate traphaving a sheathed heater embedded in the filter element for burning andremoving the collected particulates.

FIG. 11 is a schematic view of a trap and a regeneration gas feed devicemounted in the exhaust line of a diesel engine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The particulate trap for cleaning exhaust discharged from a dieselengine according to the present invention is formed of athree-dimensional mesh-like porous member as described hereinbelow.

FIGS. 2 and 3 show filter elements of different types. FIG. 2 is aperspective view of a filter element 4 in the form of a spirally woundporous sheet 10 of a heat-resistant metal and having a three-dimensionalmesh-like structure. FIG. 3 is a perspective view of a filter element 5comprising a plurality of cylindrical sheets 10 having athree-dimensional mesh-like structure and concentrically arranged one onanother.

FIGS. 4(A) and 4(B) show an embodiment in which a single filter element17 is mounted in a case. 12. FIGS. 5(A) and 5(B) show another embodimentin which a plurality of filter elements 117 are mounted in a case 112.

The particulate trap for purifying the exhaust discharged rom a dieselengine is mounted in the exhaust passage of the diesel engine to collectand remove particulates discharged from the diesel engine. It comprisesa case 12 or 112 having an exhaust gas inlet port 13 or 113 and anexhaust gas outlet port 14 or 114, and a filter element 17 or filterelements 117.

The filter elements 17 and 117 are a three-dimensional mesh-like porousmember 11 or 111 made of a heat-resistant metal and having communicatingpores. It has the shape of a spiral member 4 as shown in FIG. 2 orcomprises a plurality of concentric cylindrical layers as shown in FIG.3. The single filter element (FIG. 4) or the plurality of filterelements (FIG. 5) are mounted in the case 12 or 112, respectively.Closure members 18a, 18b or 118a, 118b are provided to close,alternately, the gap defined between the outer surface of thecylindrical member(s) and the inner surface of the case at one end ofthe casing and the opened end of the cylindrical member(s) at the otherend of the casing.

The filter element(s), of a three-dimensional mesh-like porous membermade of a heat-resistant metal, has communicating pores having anaverage diameter of 0.1-1 mm, the average number of pores as counted inthe thickness direction of the porous member being 10 or more, thevolume packing density of the metal framework to the entire filteringspace being 10-40%, and the filter having a gas inflow filtration areaof 100 cm² per liter of the displacement of the engine to which the trapis to be mounted.

FIGS. 6(A) and 6(B) show one embodiment to increase the gas inflowfiltration area of the filter element and thus to restrain the backpressure while collecting particulates and improve the particulatecollection efficiency.

In the embodiment of FIGS. 6(A) and 6(B), the particulate trap comprisesa case 212 and filter 217 in the form of three-dimensional mesh-likeporous members 211 made of a heat-resistant metal having communicatingpores 2. The filter elements 217a, 217b and 217c of the filter 217 areconcentric cylindrical members that have different diameters and arespaced apart a predetermined distance from one another. The gap definedbetween the inner surface of the case and the outermost cylindricalmember, the gaps between the adjacent cylindrical members and the openend of the innermost cylindrical member are closed alternately at thegas inlet 213 side and at the gas outlet 214 side by closure member 218,218a.

Again, the communicating pores have an average diameter of 0.1-1 mm, theaverage number of the pores as counted in the thickness direction of thefilter element being 10 or more, the volume packing density of the metalframework to the entire filtering space of the filter element being10-40%, and the filter element having a gas inflow filtration area of400 cm² or more per liter of the displacement of the engine to which thetrap is to be mounted.

FIGS. 7(A) and 7(B) show another embodiment to increase the exhaustinflow filtration area of the filter element.

The particulate trap 325 shown in FIGS. 7(A) and 7(B) comprises a case312 and a circumferentially corrugated cylindrical filter element 317mounted in the case. The filter element is a three-dimensional mesh-likeporous member 311 made of a heat-resistant metal having communicatingpores 2. Closure members 318a, 318b are provided to close the gapdefined between the outer surface of the cylindrical member and theinner surface of the case at one end of the case and to close the openend of the cylindrical member at the other end of the case.

FIGS. 8(A), 8(B) and 9(A), 9(B) show other embodiments to increase theexhaust inflow filtration area of the filter element.

The particulate traps of these embodiments comprise a case 412 or 512and an axially corrugated cylindrical filter element 425 or 525 mountedin the case. The filter element is a three-dimensional mesh-like porousmember 411 or 511 made of a heat-resistant metal having communicatingpores 2. Closure members 418 or 518 are provided to close the gapdefined between the outer surface the cylindrical member and the innersurface of the case at one end of the case and the open end of thecylindrical member at the other end of the case.

Further, the three-dimensional mesh-like porous member forming thefilter element of the present invention is made of an Ni-basedheat-resistant alloy having communicating pores. Preferably, the allowcontains Ni: 60-85% by weight and Cr: 15-50% by weight. The presentinventors have discovered that, by using a three-dimensional mesh-likeporous member made of an Ni-based heat-resistant Ni-Cr alloy as thefilter element, it can be regenerated repeatedly without the possibilityof melting or cracking when burning and removing collected particulates.This is because, by the addition of Cr element, a stable Cr oxide filmis formed when the atmospheric temperature rises to 800° C. or more byburning the collected particulates. If the content of Cr is less than15% by weight, however, such a stable oxide film will not be formed. Ifmore than 50%, an oxide film is not formed, either. Thus, the content ofCr should preferably be within the above-defined range.

Further, each filter element may be of a three-dimensional mesh-likeporous member made of an Ni-based heat resistant metal havingcommunicating pores, the porous member preferably containing Ni: 50-85%by weight, Cr: 10-50% by weight and Al: 1-6% by weight.

If the composition is out of this range, the heat resistance of thefilter element will decrease to such a level that it cannot withstandrepeated particulate collection and regeneration operations for asufficiently long period of time. By adding Al together with Cr, theheat resistance improves more markedly than when adding only Cr. Morespecifically, by adding 1% or more Al, an oxide film is more stablyformed in an oxidizing atmosphere. But if Al is added in an amount ofmore than 6%, a brittle intermetallic compound will be produced by theNi and Al elements. This worsens the workability of the filter material.Especially if one tries to bend it, it will be easily broken. If 6% orless, the workability of the filter material is kept sufficiently high,so that it can be shaped into, e.g. a cylindrical filter element with nodifficulty. An Al oxide film is highly resistant to the attack ofsulfuric acid, which is contained in the exhaust. Thus, the filterelement in the form of a three-dimensional mesh-like porous member madeof an Ni-based heat-resistant Ni-Cr-Al alloy of the above-describedcomposition shows especially high reliability for a long period of timein exhaust gas.

Further, the particulate trap according to the present invention may beprovided with an electric heater or heaters as shown in FIG. 10. Such anelectric heater may be provided on the front or rear end of the filteror on the inner or outer surface of a cylindrical filter. Further, itmay be embedded in a cylindrical filter made of a porous metal. Itsmounting position is determined taking into account the balance betweenthe power consumption and the combustion efficiency.

The present inventors conducted the following tests.

Test 1

As shown in FIG. 11, the trap according to the present invention and aregeneration gas feed device were mounted in the exhaust line of each of6- and 4-liter six-cylinder direct-injection diesel engines. The trapused here was the particulate trap shown in FIGS. 5(A) and 5(B).

The regeneration gas feed device has a light oil burner which canproduce hot air of 600°-900° C. By changing over the flow of exhaust soas to bypass the trap, the regeneration hot gas can be supplied to thetrap. In FIG. 11, numeral 101 designates the engine; 125, the trap; 102,the regeneration gas feed device; 103, the exhaust pipe; and 107, thebypass. The particulate trap 125 for purifying the exhaust of a dieselengine used in this Test comprises a filter case 116 having an inlet 113and outlet 114 for the exhaust of the diesel engine, and filter elements117 mounted in the case.

The filter elements 117 in the form of three-dimensional mesh-likeporous cylindrical members were mounted at uniform intervals in thefilter element case 112 so as to extend longitudinally in the directionof the gas flow. Due to the closure members 118a, 118b exhaust gas flowsthrough the wall to each cylindrical member 117. When the exhaust gaspasses through the walls of the filter elements, the particulatescontained in the exhaust are collected. The exhaust thus purified isdischarged from the trap.

The filter element was constructed from a three-dimensional mesh-likeporous member made of a head-resistant metal (e.g. a porous metal"Celmet" made by Sumitomo Electric Industries Ltd.). In particular, thethree-dimensional mesh-like porous member was made of a Ni-Cr alloycomprising Ni: 65% by weight and Cr: 35% by weight. It was formed byNi-electroplating a conductivity-treated foamed urethane resin havingthree-dimensional communicating pores, burning it to remove the resincomponent and yield an Ni-based material, and alloying the material bychromizing treatment. This metal sheet having a three-dimensionalmesh-like porous structure was wound spirally to form a cylindricalfilter element as shown in FIG. 2. One to seven elements were mounted ina trap case.

In order to examine how much the exhaust from a diesel engine can becleaned with the trap according to the present invention, we preparedfilters having various different structures. All of the filter elementshad a thickness of 10 mm. By changing the number of turns and the degreeof compression when forming the filter elements, the volume ratio of theporous metal in the direction of thickness of the filter elements wasvaried within the range of 5-45%. The numbers of pores in the directionof thickness were also varied.

Tables 1-3 show the structures of the filter elements used in theexperiments. Table 4-6 show the structure of comparative examples. Table7 shows the properties of the examples. Table 8 shows the properties ofthe comparative examples. Both in the examples and the comparativeexamples, we used three-dimensional mesh-like porous members made of aNi-Cr alloy having a surface roughness Rmax of 0.2 μm or more and madeby Sumitomo Electric Industries Ltd.

Without a trap, the amount of particulates discharged was 0.54 g/HP.Hr.In contrast, in the examples 1 to 14, the particulate emission rateswere 0.1 g/HP.Hr or less after 20 cycles. Moreover, it was found outthat the average particulate collection rate over 20 cycles was 60% orhigher, which meets the 1994 EPA emission standard in the United States.

Comparative examples 29, 34, 36 and 41 showed such high particulatecollection rates as to clear the EPA emission standard because theirpores had a rather small average diameter of 0.08 mm. However because ofthe small pores, the back pressure increased so high as to adverselyinfluence the engine. Although comparative examples 22, 27, 29, 34, 36and 41 showed sufficiently low particulate emission after 20 cycles, thepressure drop was too high to be acceptable. Upon completion of a20-cycle operation, the engine exhaust was directed into a bypass tofeed hot air heated to an average of 700° C. into the trap from theregeneration hot gas feed device for about 15 minutes at the feed rateof 2 m³ /min. The collected particulates were burned by the hot air andthe filter was regenerated. After regeneration of the filter, thepressure drop decreased sharply to approximately the initial pressure,i.e. 1-2 KPa. This clearly shows that the collected particulates hadbeen burned and removed and thus the exhaust filter had been regeneratedalmost thoroughly. After regeneration, the particulate filter sufferedno melting, cracks or extreme oxidation or corrosion. Exhaust gas wasintroduced again into the filter thus regenerated. After 20 cycles, theexhaust circuit was changed over again to regenerate the filter byfeeding hot gas to it.

This collection/regeneration test was further kept up until regenerationwas repeated 300 times. Even at this point, no significant increase orchange in pressure drop was observed with the trap of the presentinvention. Also, even when the regeneration conditions were notcontrolled strictly, the filter never damaged. Moreover, even becameafter the filter had been subjected to regeneration 300 times, thepressure drop immediately after regeneration was kept at substantiallythe initial pressure, i.e. 1-2 KPa. After the filter had beenregenerated 100 times, neither external damage such as melting andcracks nor mechanical deterioration was observed.

Also, the temperatures at a plurality of points of the filter elementwere measured during regeneration. The maximum temperature reached 850°C. but the temperature dropped within three minutes at the longest. Thismeans that the collected particles had been burned off in three minutes.

The filter elements used in the above experiments had such aconstruction that the exhaust flowed from the outside to inside of thefilter elements through their wall. However even with traps which differfrom the traps used in the above experiments only in that the exhaust isadapted to flow from the inside to outside of the filter elements, theparticulates in the exhaust were collected at the rate of 60% or moreand the concentration of particulates in the exhaust was 0.08 g/HP.Hr orless, which meets the emission standard. Also, after regenerated 300times, the filter elements suffered no damage and the amount ofparticulates in the exhaust was within the EPA standard.

In the Test 1, one to seven cylindrical filter elements were mounted ina filter element case. Instead, the particulate trap 225 having thefilter 217 shown in FIGS. 6(A), 6(B) may be used. The filter 217comprises a plurality of cylindrical filter elements 217a, 217b and 217chaving different diameters from one another and formed by winding athree-dimensional mesh-like porous member 10 made of a heat-resistantmetal having communicating pores. With this arrangement, it is possibleto increase the gas inflow filtration area per liter of the enginedisplacement compared with the traps used in Test 1 provided the casesused are of the same size.

Further, the embodiment of FIGS. 6(A), 6(B), may be mounted in theopposite way with the portion 213 located on the exhaust outlet side andthe portion 214 on the inlet side. In this arrangement, the exhaustflows from inside to outside of the filter elements. The particulatecollection rate decreased about 10% in this case but was stillsufficiently high.

Test 2

Projected Width of the Section of the Porous Structure

We examined the relation between the projected width of the section ofthe metal skeleton forming the three-dimensional mesh-like porous memberand the characteristics of the filter. In the test, we used the traphaving the filter elements shown in FIGS. 5(A), 5(B) and mounted in theexhaust line of a 2.8-liter swirl-chamber diesel engine. The sections ofthe metal skeleton in the examples and the comparative examples haddifferent projected widths. After driving the engine for three hours at1800 rpm and 5 kgf.m, the amount of the collected particulates and thepressure drop were measured. The section of the metal skeleton formingthe three-dimensional mesh-like porous member had a projected width of17-250 μm, though this figure changes according to the average diameterand the number of pores. Table 9 shows the structures of the particulatetraps of the examples and the comparative examples used in this Test.

Table 10 shows the results of Test 2. It was confirmed that thethree-dimensional mesh-like porous members of the example can collectsufficient amounts of particulates without unduly increasing thepressure drop, provided that the pores in the porous member have adiameter of 0.1-1.0 mm, that the average number of pores as counted inthe thickness direction of the filter element is 10 or more, that thevolume packing density of the porous member in the entire filteringspace is 10-40%, that the exhaust inflow filtration area of the filteris 400 cm² or more, and that the projected width of section of the metalskeleton to be formed is 20 μm or more. If any of the above conditionsis not met, the particulate collection capacity was not high enough andthe pressure drop was not low enough.

Test 3

Surface Roughness

We examined the relation between the surface condition of thethree-dimensional mesh-like porous member and the filtering properties.

The test was conducted by mounting the trap in the exhaust line of a2.8-liter diesel engine. The engine was driven for three hours at 1800rpm with the torque of 6 kgf.m to collect particulates.

In the test, we used a trap having concentrically arranged filterelements shown in FIGS. 6(A), 6(B).

The filter elements used in the experiments were formed by rolling asheet of the three-dimensional mesh-like porous material with apredetermined radius of curvature. The sizes of the filter elements weredetermined taking into account the gas inflow area of the filterelements as shown in Test 1. More specifically, a continuous sheet wasformed from a three-dimensional mesh-like porous member having anaverage pore diameter of 0.5 mm. This sheet was formed into acylindrical member having an outer diameter of 140 mm and a thickness of10 mm, a cylindrical member having an outer diameter of 110 mm and athickness of 10 mm, and a cylindrical member having an outer diameter of80 mm and a thickness of 10 mm. The volume packing density of the metalskeleton to the entire filter space was 12.5%. The effective length ofthe filter elements was 150 mm. Closure members and presser members wereused to secure and close the ends of the filter elements so that exhaustmay flow as shown in FIG. 6(A).

The heat-resistant porous member used in the experiment was a porousNi-Cr alloy formed by chromizing an Ni-based three-dimensional mesh-likeporous member made by Sumitomo Electric Industries.

In the chromizing treatment, chromium chloride gas is produced from thepowder. Thus, by controlling the gas producing amount or the speed withwhich it deposits onto the metal skeleton, the surface condition of theNi-Cr alloyed metal porous member can be changed freely as in a typicalCVD process. Such a powder-alloying technique is also used when formingan Ni-Cr-Al alloyed metal porous member. It is well-known that a smoothsurface can be obtained if the material gas is produced at a slow rateand deposited on the surface of Ni-based porous member and that if thematerial gas is produced at high speed, a rough surface results. Even ifthe surface is rough immediately after deposition, it will be smoothedgradually by leaving the porous member at a high temperature of 1000° C.or more in a reducing atmosphere because in such an atmosphere, thedeposited elements disperse into the metal skeleton.

In this Test, we examined the properties of various filters formed underdifferent deposition/dispersion conditions so that they have differentdegrees of surface roughness. For the purpose of comparison, we alsoprepared porous filters as comparative examples. They were subjected todispersion treatment at 1050° C. for 20 hours, which is longer than fivetimes the normal time for such treatment, to obtain a smooth surfacehaving an Rmax value of 0.1 μm or less.

Table 11 shows the particulate collection amounts of these filterelements as a function of time. As shown in Table 11, in the initialstage, the filters having rougher surfaces can collect particulates withhigher efficiency than those having smoother surfaces. It is clear fromthis fact that there is a close correlation between the surfaceroughness of the filter and its particulate collection capacity,especially in the initial stage of particulate collection. We found thatthe filter can collect particulates at a sufficiently high rate if ithas a surface roughness of 0.2 μm or higher in Rmax. The shape of thefilter of the present invention is not limited only if its surfaceroughness is 0.2 μm or more in Rmax. Also, it may be made of Ni-Cr-Alalloyed metallic porous material, which is formed by powder-alloying aswith an Ni-Cr alloyed metallic porous material or any other material.

Test 4

Corrugated Filter

The embodiment of the trap of FIGS. 7(A), 7(B) was used in this Test.

Examples

We prepared the circumferentially corrugated cylindrical filter elementby press-forming a sheet cut to a predetermined size and made of athree-dimensional mesh-like porous metal containing 40% by weight of Cr.The element had a constant thickness of 10 mm. In order to make thethickness constant, it was formed by laminating corrugated sheets ofdifferent sizes, bonding them together and reforming the shape. The casehad an inner diameter of 160 mm. The filter element was 350 mm long.

In examples shown in Table 12, the three-dimensional mesh-like porousmembers had pores having diameters of 0.1, 0.5 and 1.0 mm. Theirthicknesses were adjusted so that the volume packing densities will bewithin the range of 10-40%. The number of pores was counted in thethickness direction at three points.

The corrugated cylindrical filter element 317 thus formed was mounted inthe filter element case 312. Closure members were provided to close oneend of the gap defined between the outer surface of the cylindricalmember 317 and the inner surface of the case 312 and the opening of thecylindrical member at the other end.

Comparative Examples

As comparative examples, we prepared a particulate trap comprising thesame case as used in the examples and a corrugated filter element whichis of the same type as that of the examples but has fewer, i.e. fourcorrugations, and a trap having a cylindrical uncorrugated filterelement. Both filter elements have their ends closed. Specificstructures of the comparative examples as well as the examples are shownin Table 12.

The test was conducted by mounting the respective traps in the exhaustline of a 2.8-liter diesel engine. The engine was driven for six hoursat 1600 rpm with the torque of 6 kgf.m to collect particulates. Theparticulate collection capacity and the pressure drop were measured foreach of the examples and the comparative examples.

As shown in Table 13, the particulate traps of the examples couldcollect substantially the same amount of particulates in a predeterminedperiod of time, while keeping the pressure drop lower than thecomparative examples by 15-20%.

From the results of Test 4, it was found out that by using a trap havinga corrugated filter element, the pressure drop can be kept low comparedwith a trap having a completely cylindrical filter element of the samediameter, provided the cases have the same exhaust inflow filtrationarea and the filter elements have the same particulate collectioncapacity.

As shown in FIG. 7(A), exhaust gas introduced into the filter elementsused in the test flows from the outside of the filter element to itsinside. However particulates can be collected with sufficiently highefficiency even if the end faces are closed in the opposite way so thatthe exhaust flows from inside to outside. It was confirmed in a separatetest that the collection efficiency of such a trap decreases only about10% provided it is of the same size.

Test 5

The embodiment of the trap shown in FIGS. 8(A), 8(B) was used for thisTest. The filter element was a three-dimensional mesh-like porous member411 made of a heat-resistant Ni-Cr alloy metal having communicatingpores.

Examples

The present inventors prepared axially corrugated cylindrical filterelements by press-forming a sheet having a predetermined size and madeof a three-dimensional mesh-like porous metal containing 40% by weightof Cr. The elements had a constant thickness of 10 mm. In order to makethe thickness constant, they were formed by laminating corrugated sheetsof different sizes, bonding them together and reforming the shape. Thecases had an inner diameter of 160 mm as with the cases used in Test 4.The filter elements were 350 mm long. The filter elements had three,four and seven axial corrugations, respectively, and had different gasinflow filtration areas. Their performance was evaluated.

Comparative Examples

For comparison purposes, the inventors prepared particulate traps havingcompletely cylindrical uncorrugated filter elements. The size of thecases and the length and thickness of the filter elements were the sameas those of the examples.

Specific structures of the comparative examples as well as the examplesare shown in Table 14.

The test was conducted by mounting the respective traps in the exhaustline of a 2.8-liter diesel engine. The engine was driven for six hoursat 1600 rpm with the torque of 6 kgf.m to collect particulates. Theparticulate collection capacity and the pressure drop were measured foreach of the examples and the comparative examples.

Table 15 shows the results of the test.

It was found out that an axially corrugated filter element could lowerthe pressure drop, although the particulate collection capacity issubstantially unchanged. This is because the gas inlet filtration areaof the filter element forming the particulate trap increases bycorrugating it.

As shown in FIG. 8(A), exhaust gas introduced into the filter elementsused in this Test flows from the outside of the filter element to itsinside. However particulates can be collected with sufficiently highefficiency even if the end faces are closed in the opposite way so thatthe exhaust flows in the opposite way.

Test 6

The filter used in Test 1 was a three-dimensional mesh-like porousmember made of an Ni-Cr alloy. But the composition of such an Ni-Cralloy is not limited to the one shown in Test 1. The inventors preparedfilters made of Ni-Cr alloys having different compositions. They alsoprepared three-dimensional mesh-like porous filter elements made ofNi-Cr-Al alloys. The filters were regenerated by burning the collectedparticulates with hot air heated to an average temperature of 800° C. bya burner and fed from the regeneration hot gas feed device at the feedrate of 2 m³ /min.

Table 16 shows the test results of the examples and the comparativeexamples. As shown in Table 16, the three-dimensional mesh-like porousmembers made of Ni-Cr alloys having the compositions shown in the tableand the three-dimensional mesh-like porous members made of Ni-Cr-Alalloys having the compositions shown in the table never suffered damageeven after they had been regenerated 500 times, and thus showedsufficient durability.

Specifically, the filters made of Ni-Cr alloys containing 15% by weightor more Cr were durable enough not to develop any large amount of damageeven after they had been regenerated 500 times. The filters containing10% by weight or less Cr was brittle and developed porous oxide scales.Thus, they were not durable enough. It was also found out that theaddition of Al cannot significantly improve the heat resistance of thefilters.

Test 7

Heater

In Test 1, as one example of the method for removing the particulatescollected by the filter element, the heat produced by the burner withoutany strict heat control was used to burn the particulates collected. Inthis Test, the particulates collected by the porous member were burnedand removed with an electric heater.

The mounting position of the heater element is important in this case.As an example, as shown in FIG. 10, the inventors used a cylindricalfilter element having sheathed heaters made of a three-dimensionalmesh-like porous material. The sheathed heaters are embedded in the wallof the filter element for collecting particulates when exhaust gas flowstherethrough. In FIG. 10, the sheathed heaters 622A-622D are embedded inthe cylindrical member 617. They are disposed as near to the gas inletas possible. The sheathed heaters 622A and 622B are each a one-piecemember which is bent near the end face B. The bent portion is embeddedin the end face B. The heaters 622C and 622D have the same structure asthe heaters 622A and 622B. A heaters 622A-622D are arranged at the pitchof 90°.

Four such filter elements with heaters were mounted in the trap shown inFIGS. 5(A), 5(B) with their end faces closed. In this Test, the filterelements were mounted in the trap such that the exhaust flows from theinside to the outside of each filter element. Each filter element had anouter diameter of 150 mm, a thickness of 10 mm was 350 mm long. Thefilter elements were three-dimensional mesh-like porous members made ofan Ni-Cr alloy and having pores having an average diameter of 0.5 mm.The number of pores as counted in the thickness direction was 35 andtheir volume packing density to the entire filtering space was 20%. Thistrap was mounted in the discharge line of a 2.0-liter diesel engine andthe engine was driven at 2000 rpm with a torque of 5 kgf.m. The initialpressure drop was 1 KPa. As the engine was driven, the amount ofparticulates collected and thus the pressure drop increased gradually.When the engine was driven five hours, the pressure drop increased to 6KPa. It is necessary to burn and remove the particulates discharged fromthe diesel engine by activating the electric heaters before the backpressure unduly increases. In this Test, the heaters were activated byconnecting the ends 622A and 622B to the (+) terminal of a power sourceand the ends 622C and 622D to the (-) terminal. As the heaters wereoperated, the pressure drop of the trap decreased sharply to the initiallevel of 1-2 KPa. This clearly shows that the collected particulates hadbeen burned and removed and the filters had been regenerated properly.

The collected particulates are heated by the heaters, which are providednear the exhaust inflow side, together with the metallicthree-dimensional mesh-like porous members, so that the particulatesquickly ignite and burn off. The filters are thus regenerated quickly.

Observation of the filters thus regenerated revealed nothing abnormal.This shows that, with this arrangement, the filter material is kept freeof undue loads.

The three-dimensional mesh-like material used in the present inventionis a porous metal and thus has a small heat capacity and exhibits lowheat conductivity compared with cordierite filters. Thus, the heatproduced in the area where the soot is burned is discharged quicklytogether with a large amount of exhaust gas. Thus, the filters are lesslikely to be overheated locally. Since the filters are heated only inthe limited area where the particulates are burned, no sharp burning orthermal shock due to cooling occurs. Rather, the filters can be heatedat a moderate rate.

In this Test, the heaters are embedded in the cylindrical member nearits inner surface at the gas inlet side. But if the exhaust gas is to beintroduced from outside to inside of the cylindrical member, the heatersshould preferably be embedded near the outer surface of the cylindricalmember in view of heat efficiency.

In this Test, four filter elements were mounted in the trap as shown inFIG. 5(B). Of course, the number of filter elements may be variedaccording to the engine displacement and the structure of theregeneration system.

According to the present invention, there is provided a particulate trapfor purifying diesel engine exhaust which can collect the particulatesin the exhaust with high efficiency while not unduly increasing thepressure drop and which can sufficiently withstand thermal stressesduring the regeneration treatment.

                                      TABLE 1                                     __________________________________________________________________________    Structure of particulate traps of the present invention used in Test 1                      Example                                                                       1   2   3   4   5   6   7                                       __________________________________________________________________________    Size of cylindrical member                                                    Outer diameter (mm)                                                                         250 250 250 250 250 250 250                                     Inner diameter (mm)                                                                         230 230 230 230 230 230 230                                     Length (mm)   300 300 300 300 460 460 460                                     Number        1   1   1   1   1   1   1                                       Average pore diameter (mm)                                                                  0.1 0.5 1.0 0.5 0.1 0.5 1.0                                     Average numbers of pores in                                                                 120 27  12  50  145 35  21                                      direction of thickness                                                        Volume packing density (%)                                                                  10  12  10  40  10  20  25                                      Gas inflow filtration area                                                                  400 400 400 400 600 600 600                                     per liter of exhaust (cm.sup.2)                                               __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Structure of particulate traps of the present invention used in Test 1                      Example                                                                       8   9   10  11  12  13  14                                      __________________________________________________________________________    Size of cylindrical member                                                    Outer diameter (mm)                                                                         100 100 100 100 100 100 100                                     Inner diameter (mm)                                                                         80  80  80  80  80  80  80                                      Length (mm)   190 190 190 190 290 290 290                                     Number        4   4   4   4   4   4   4                                       Average pore diameter (mm)                                                                  0.1 0.5 1.0 0.5 0.1 0.5 1.0                                     Average number of pores in                                                                  120 27  12  50  145 35  21                                      direction of thickness                                                        Volume packing density (%)                                                                  10  12  10  40  10  20  25                                      Gas inflow filtration area                                                                  400 400 400 400 600 600 600                                     per liter of exhaust (cm.sup.2)                                               __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Structure of particulate traps of the present invention used in Test 1                      Example                                                                       15  16  17  18  19  20  21                                      __________________________________________________________________________    Size of cylindrical member                                                    Outer diameter (mm)                                                                         80  80  80  80  80  80  80                                      Inner diameter (mm)                                                                         60  60  60  60  60  60  60                                      Length (mm)   140 140 140 140 200 200 200                                     Number        7   7   7   7   7   7   7                                       Average pore diameter (mm)                                                                  0.1 0.5 1.0 0.5 0.1 0.5 1.0                                     Average number of pores in                                                                  120 27  12  50  145 35  21                                      direction of thickness                                                        Volume packing density (%)                                                                  10  12  10  40  10  20  25                                      Gas inflow filtration area                                                                  400 400 400 400 600 600 600                                     per liter of exhaust (cm.sup.2)                                               __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________    Structure of particulate traps of the present invention used in Test 1                      Example                                                                       22  23  24  25  26  27  28                                      __________________________________________________________________________    Size of cylindrical member                                                    Outer diameter (mm)                                                                         250 250 250 250 250 250 250                                     Inner diameter (mm)                                                                         230 230 240 220 230 240 230                                     Length (mm)   270 270 270 350 350 460 460                                     Number        1   1   1   1   1   1   1                                       Average pore diameter (mm)                                                                  0.08                                                                              1.2 1.2 1.2 0.5 0.08                                                                              1.2                                     Average number of pores in                                                                  140 25  9   20  56  70  23                                      direction of thickness                                                        Volume packing density (%)                                                                  10  40  5   12  45  8   42                                      Gas inflow filtration area                                                                  350 350 350 450 450 600 600                                     per liter of exhaust (cm.sup.2)                                               __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________    Structure of particulate traps of the comparative examples used in Test                     Comparative example                                                           29  30  31  32  33  34  35                                      __________________________________________________________________________    Size of cylindrical member                                                    Outer diameter (mm)                                                                         100 100 100 100 100 100 100                                     Inner diameter (mm)                                                                         80  80  80  80  80  90  80                                      Length (mm)   170 170 170 190 290 290 290                                     Number        4   4   4   4   4   4   4                                       Average pore diameter (mm)                                                                  0.08                                                                              1.2 0.5 1.2 0.5 0.08                                                                              1.2                                     Average number of pores in                                                                  140 25  23  20  56  70  22                                      direction of thickness                                                        Volume packing density (%)                                                                  10  45  6   22  45  8   42                                      Gas inflow filtration area                                                                  350 350 350 400 600 600 600                                     per liter of exhaust (cm.sup.2)                                               __________________________________________________________________________

                                      TABLE 6                                     __________________________________________________________________________    Structure of particulate traps of the comparative examples used in Test                     Comparative example                                                           36  37  38  39  40  41  42                                      __________________________________________________________________________    Size of cylindrical member                                                    Outer diameter (mm)                                                                         80  80  80  80  80  80  80                                      Inner diameter (mm)                                                                         60  60  70  60  60  70  60                                      Length (mm)   120 120 120 140 140 205 205                                     Number        7   7   7   7   7   7   7                                       Average pore diameter (mm)                                                                  0.08                                                                              1.2 0.5 1.2 0.5 0.08                                                                              1.2                                     Average number of pores in                                                                  140 25  23  20  56  70  22                                      direction of thickness                                                        Volume packing density (%)                                                                  10  45  6   22  45  8   42                                      Gas inflow filtration area                                                                  350 350 350 400 400 600 600                                     per liter of exhaust (cm.sup.2)                                               __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    Evaluation result of the examples in Test 1                                                         After re-                                                                     generating                                                                          After 100 collections                                                                      After 300 collections                                      after 20                                                                            & regenerating repeated                                                                    & regenerating repeated                         After 20 cycles of                                                                       collect-                                                                            After After  After After                          Initial    collections                                                                              ions  collecting                                                                          regenerating                                                                         collecting                                                                          regenerating                   Pressure   Collection                                                                          Pressure                                                                           Pressure                                                                            Pressure                                                                            Pressure                                                                             Pressure                                                                            Pressure                       drop       efficiency                                                                          drop drop  drop  drop   drop  drop                           (KPa)      (%)   (KPa)                                                                              (KPa) (KPa) (KPa)  (KPa) (KPa)                          __________________________________________________________________________    Example 1                                                                           1.7  85    28   1.7   28    1.8    28    1.0                            Example 2                                                                           1.4  70    22   1.3   22    1.2    22    1.2                            Example 3                                                                           1.0  69    14   1.0   15    1.0    15    1.0                            Example 4                                                                           1.3  75    25   1.3   26    1.4    26    1.4                            Example 5                                                                           1.5  85    27   1.5   27    1.5    27    1.5                            Example 6                                                                           1.0  70    20   1.0   20    1.0    20    1.0                            Example 7                                                                           1.1  63    18   1.1   20    1.1    20    1.1                            Example 8                                                                           1.8  85    28   1.7   28    1.8    28    1.8                            Example 9                                                                           1.4  72    21   1.4   21    1.4    21    1.4                            Example 10                                                                          1.0  61    15   1.0   15    1.0    15    1.0                            Example 11                                                                          1.6  78    25   1.6   28    1.6    28    1.6                            Example 12                                                                          1.9  83    26   2.0   27    2.0    27    2.0                            Example 13                                                                          1.3  65    27   1.4   25    1.4    25    1.4                            Example 14                                                                          1.0  65    18   1.0   18    1.0    18    1.0                            Example 15                                                                          1.8  87    28   1.8   28    1.8    28    1.8                            Example 16                                                                          1.5  76    25   1.5   25    1.5    25    1.5                            Example 17                                                                          1.0  62    20   1.0   20    1.0    20    1.0                            Example 18                                                                          2.0  85    29   2.0   28    2.0    28    2.0                            Example 19                                                                          1.8  80    26   1.8   26    1.8    26    1.8                            Example 20                                                                          1.7  81    27   1.8   27    1.7    27    1.7                            Example 21                                                                          1.2  70    20   1.2   20    1.2    20    1.2                            __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    Evaluation result of the comparative examples in Test 1                                              After re-                                                                     generating                                                                          After 100 collections                                                                      After 300 collections                                      after 20                                                                            & regenerating repeated                                                                    & regenerating repeated                         After 20 cycles of                                                                       collect-                                                                            After After  After After                                Initial                                                                            collections                                                                              ions  collecting                                                                          regenerating                                                                         collecting                                                                          regenerating                         Pressure                                                                           Collection                                                                          Pressure                                                                           Pressure                                                                             Pressure                                                                           Pressure                                                                             Pressure                                                                            Pressure                             drop efficiency                                                                          drop drop   drop drop   drop  drop                                 (KPa)                                                                              (%)   (KPa)                                                                              (KPa)  (KPa)                                                                              (KPa)  (KPa) (KPa)                         __________________________________________________________________________    Compara- 22                                                                          2.0  75    45   2.0   47    2.0    47    2.0                           tive 23                                                                              1.6  53    32   1.6   34    1.6    34    1.6                           example 24                                                                           1.3  40    25   1.3   27    1.3    27    1.3                           25     1.4  50    25   1.4   25    1.4    25    1.4                           26     2.0  65    42   2.0   26    2.0    26    2.0                           27     1.9  70    37   1.9   38    1.9    38    1.9                           28     1.6  55    28   1.6   28    1.6    28    1.6                           29     2.1  73    46   2.1   45    2.1    45    2.1                           30     1.6  50    28   1.7   28    1.6    28    1.6                           31     1.2  45    28   1.3   28    1.2    28    1.2                           32     1.2  55    28   1.3   29    1.2    29    1.2                           33     2.2  65    46   2.2   48    2.2    48    2.2                           34     2.0  70    39   2.0   38    2.0    38    2.0                           35     1.4  53    26   1.4   27    1.4    27    1.4                           36     2.1  76    48   2.2   45    2.1    45    2.1                           37     1.6  52    30   1.7   31    1.6    31    1.6                           38     1.2  45    28   1.2   28    1.2    28    1.2                           39     1.2  40    24   1.3   24    1.2    24    1.2                           40     2.2  67    48   2.3   47    2.2    47    2.2                           41     2.0  70    38   2.0   38    2.0    38    2.0                           42     1.5  52    28   1.5   28    1.5    28    1.5                           __________________________________________________________________________

                                      TABLE 9                                     __________________________________________________________________________    Structure of particulate traps used in Test 2                                 (Projected width of skeleton section)                                                                           Comparative                                                   Example         example                                                       43  44  45  46  47  48  49                                  __________________________________________________________________________    Size of cylindrical member                                                    Outer diameter (mm)                                                                             140 70  70  70  70  70  70                                  Inner diameter (mm)                                                                             120 50  50  50  50  50  50                                  Length (mm)       350 325 325 325 325 325 150                                 Number of cylindrical member                                                                    1   4   4   4   4   4   4                                   Average pore diameter (mm)                                                                      1.0 0.5 0.5 0.1 0.08                                                                              1.2 0.08                                Projected width of framework (μm)                                                            200 100 60  20  17  250 17                                  Average numbers of pores in                                                                     17  28  27  125 140 18  200                                 direction of thickness                                                        Volume packing density (%)                                                                      25  25  25  10  10  30  25                                  Gas inflow filtration area                                                                      600 600 600 600 600 600 400                                 per liter of exhaust (cm.sup.2)                                               __________________________________________________________________________

                  TABLE 10                                                        ______________________________________                                        Evaluation result in Test 2                                                           Amount of particulate collected (g)                                           and pressure drop (KPa)                                                         Before    20 minutes                                                                              1 hour 3 hours                                  No.       collecting                                                                              later     later  later                                    ______________________________________                                        Example 43                                                                              0      0.4    0.25 0.6  1.2  1.0 4.0  3.8                           Example 44                                                                              0      0.5    0.40 0.9  2.0  2.0 5.2  5.5                           Example 45                                                                              0      0.5    0.35 0.8  1.6  1.8 6.0  5.6                           Example 46                                                                              0      0.6    0.5  1.2  2.1  2.1 5.9  5.7                           Comparative                                                                             0      0.5    0.5  1.1  2.2  2.9 6.2  8.0                           example 47                                                                    Comparative                                                                             0      0.5    0.2  0.9  0.8  1.8 2.2  3.5                           example 48                                                                    Comparative                                                                             0      0.7    0.6  1.5  2.2  3.0 6.0  15.2                          example 49                                                                    ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                        Evaluation result of skeleton surface and collection performance              of three-dimensional porous members in Test 3                                             Amount of particulate collected (g) and                                       pressure drop (KPa)                                                                 Before  20                                                                    collect-                                                                              minutes                                                                              60 minutes                                                                            3 hours                              No.               ing     later  later   later                                ______________________________________                                        Comparative                                                                            0.1 μm<                                                                             0     0.5 0.25 0.5 0.6  1.0  2.0 1.8                        example 50                                                                    Example 51                                                                             0.2 μm                                                                              0     0.5 0.5  1.1 1.2  1.8  3.5 2.5                        Example 52                                                                             0.5 μm                                                                              0     0.5 0.6  1.2 1.2  1.9  3.6 2.6                        Example 53                                                                             1.0 μm                                                                              0     0.5 0.7  1.3 1.3  2.1  3.7 2.7                        ______________________________________                                    

                                      TABLE 12                                    __________________________________________________________________________    Structure of particulate traps of the present invention and                   the comparative examples used in Test 4 (waveform 1, direction of             circle)                                                                                                      Comparative                                                   Example         example                                                       54  55  56  57  58  59  60                                     __________________________________________________________________________    Size of cylindrical member                                                    Maximum outer diameter (mm)                                                                  140 140 140 140 150 140 140                                    Minimum inner diameter (mm)                                                                  50  50  50  50  130 50  50                                     Thickness (mm) 10  10  10  10  10  10  10                                     Length (mm)    350 350 350 350 350 350 350                                    Number of pitches                                                                            6   6   6   6   0   4   4                                      Average pore diameter (mm)                                                                   0.1 0.5 1.0 0.5 0.5 0.08                                                                              1.2                                    Average number of pores in                                                                   120 25  13  35  42  140 15                                     direction of thickness                                                        Volume package density (%)                                                                   20  12  8   33  42  8   42                                     Gas inflow filtration area                                                                   700 700 700 700 550 550 550                                    per liter of exhaust (cm.sup.2)                                               __________________________________________________________________________

                  TABLE 13                                                        ______________________________________                                        Evaluation result in Test 4                                                           Amount of particulate collected (g) and                                       pressure drop (KPa)                                                             Before      1 hour     6 hours                                      No.       collecting  later      later                                        ______________________________________                                        Example 54                                                                              0       0.6     1.6  1.4   7.5   8.5                                Example 55                                                                              0       0.5     1.4  1.2   7.4   8.0                                Example 56                                                                              0       0.4     1.2  1.0   7.4   7.5                                Example 57                                                                              0       0.6     1.4  1.1   7.4   8.5                                Comparative                                                                             0       0.6     1.4  1.5   7.3   12.0                               example 58                                                                    Comparative                                                                             0       0.6     1.5  2.0   7.4   15.2                               example 59                                                                    Comparative                                                                             0       0.4     1.2  1.4   7.2   10.1                               example 60                                                                    ______________________________________                                    

                                      TABLE 14                                    __________________________________________________________________________    Structure of particulate traps of the present invention and                   the comparative examples used in Test 4 (waveform 1, direction of             circle)                                                                                                  Comparative                                                       Example     example                                                           61  62  63  64  65  66                                         __________________________________________________________________________    Size of cylindrical member                                                    Maximum outer diameter (mm)                                                                  140 140 140 140 150 150                                        Minimum inner diameter (mm)                                                                  50  50  50  120 130 130                                        Length (mm)    350 350 350 350 350 350                                        Thickness (mm) 10  10  10  10  10  10                                         Number of pitches                                                                            3   5   7   0   0   0                                          Average pore diameter (mm)                                                                   0.5 0.5 0.5 0.5 0.5 0.08                                       Average number of pores in                                                                   27  27  20  35  8   140                                        direction of thickness                                                        Volume package density (%)                                                                   12  12  12  20  10  8                                          Gas inflow filtration area                                                                   440 570 720 400 600 600                                        per liter of exhaust (cm.sup.2)                                               __________________________________________________________________________

                  TABLE 15                                                        ______________________________________                                        Evaluation result in Test 5                                                           Amount of particulate collected (g) and                                       pressure drop (KPa)                                                             Before     1 hour      6 hours                                      No.       collecting later       later                                        ______________________________________                                        Example 61                                                                              0       0.5    1.5   1.4   7.2   8.5                                Example 62                                                                              0       0.5    1.6   1.2   7.3   8.0                                Example 63                                                                              0       0.5    1.5   1.0   7.2   7.1                                Comparative                                                                             0       0.6    1.5   1.6   7.3   11.2                               example 64                                                                    Comparative                                                                             0       0.5    1.5   1.5   7.1   10.5                               example 65                                                                    Comparative                                                                             0       0.7    1.7   2.0   7.0   13.0                               example 66                                                                    ______________________________________                                    

                  TABLE 16                                                        ______________________________________                                        Composition of three-dimensional mesh-like porous members used                in Test 6 and skeleton deterioration after regenerating                               Composition of porous                                                                        Framework surface                                              member (wt %)  after 500                                              No.       Ni      Cr      Al     regenerations                                ______________________________________                                        Example 67                                                                              50      50      --     No damage                                    Example 68                                                                              80      20      --     No damage                                    Example 69                                                                              65      35      --     No damage                                    Example 70                                                                              75      24      1      No damage                                    Example 71                                                                              85      12      6      No damage                                    Example 72                                                                              50      44      6      No damage                                    Example 73                                                                              70      24      6      No damage                                    Comparative                                                                             87      13      --     Brittle scale                                example 74                       appeared                                     Comparative                                                                             70      20      10     Not cylindrically                            example 75                       workable                                     Comparative                                                                             60      39.5    0.5    Brittle scale                                example 76                       appeared                                     Comparative                                                                             80      10      10     Not workable                                 example 77                                                                    Comparative                                                                             70      29.5    0.5    Brittle scale                                example 78                       appeared                                     ______________________________________                                    

We claim:
 1. The combination of a diesel engine having an exhaust line,and a particulate trap for filtering particulates from exhaust gas ofthe diesel engine, said particulate trap including a case having aninlet and an outlet mounted to said exhaust line of the diesel enginesuch that said particulate trap is disposed in-line with said exhaustline, and a filter comprising at least one filter element mounted insaid case, each said filter element being a porous member of aheat-resistant metal framework having the form of a three-dimensionalmesh, said porous member being tubular so as to have an outer surfaceand an inner surface, the pores of said porous member definingpassageways extending between and open to said inner and said outersurfaces, said porous member extending longitudinally in a direction inwhich exhaust gas flowing through said exhaust line passes from saidinlet to said outlet of the case, the surface roughness of the metal ofsaid metal framework being at least 0.2 μm as expressed in terms ofRmax, the pores of said porous member having an average diameter of0.1-1 mm, the average number of said pores as counted in the directionof thickness of said porous member being at least 10, the amount ofspace occupied by said metal framework to the entire volume of spaceoccupied by said porous member being 10-40%, and said porous memberhaving a gas inflow filtration area of 400 cm² or more per liter of thedisplacement of said diesel engine.
 2. The combination of a dieselengine and particulate trap as claimed in claim 1, wherein the projectedwidth of the sections of said metal framework between adjacent ones ofsaid pores is at least 20 μm.
 3. The combination of a diesel engine andparticulate trap as claimed in claim 1, wherein said filter comprises aplurality of said filter elements spaced from one another side-by-sidein the case, and said trap also includes closure members, said closuremembers closing each of the ends of said filter elements closest to oneof said inlet and said outlet of the case in said direction in whichexhaust gas flows through said case, and said closure members closing agap between an inner surface of said case and other ends of the filterelements closest to the other of said inlet and said outlet in saiddirection in which exhaust gas flows through said case.
 4. Thecombination of a diesel engine and particulate trap as claimed in claim1, wherein said filter comprises a plurality of said filter elementsdisposed concentrically in the case and spaced from one another so as todefine gaps therebetween, and said trap also includes closure members,said closure members closing alternate ones of a gap between an innersurface of said case and an end of the radially outermost one of theconcentric filter elements, gaps between adjacent ones of said filterelements, and an end of the radially innermost one of said filterelements, at each of the ends of said porous members, only one end ofthe radially innermost filter element being closed by said closuremembers.
 5. The combination of a diesel engine and particulate trap asclaimed in claim 1, wherein said filter element has a corrugatedsectional shape, and said trap includes closure members, said closuremembers closing one end of said filter element closest to one of saidinlet and said outlet in the direction in which exhaust gas flowsthrough said case, and said closure members closing a gap between aninner surface of said case and the other end of said filter elementclosest to the other of said inlet and said outlet in said direction inwhich exhaust gas flows through said case.
 6. The combination of adiesel engine and particulate trap as claimed in claim 1, wherein saidheat-resistant metal is an Ni-based alloy containing Ni: 50-80% byweight and Cr: 20-50% by weight.
 7. The combination of a diesel engineand particulate trap as claimed in claim 2, wherein said heat-resistantmetal is an Ni-based alloy containing Ni: 50-80% by weight and Cr:20-50% by weight.
 8. The combination of a diesel engine and particulatetrap as claimed in claim 3, wherein said heat-resistant metal is anNi-based alloy containing Ni: 50-80% by weight and Cr: 20-50% by weight.9. The combination of a diesel engine and particulate trap as claimed inclaim 4, wherein said heat-resistant metal is an Ni-based alloycontaining Ni: 50-80% by weight and Cr: 20-50% by weight.
 10. Thecombination of a diesel engine and particulate trap as claimed in claim5, wherein said heat-resistant metal is an Ni-based alloy containing Ni:50-80% by weight and Cr: 20-50% by weight.
 11. The combination of adiesel engine and particulate trap as claimed in claim 1, wherein saidheat-resistant metal is an Ni-based alloy containing Ni: 50-85% byweight, Cr: 15-50% by weight and Al: 1-6% by weight.
 12. The combinationof a diesel engine and particulate trap as claimed in claim 2, whereinsaid heat-resistant metal is an Ni-based alloy containing Ni: 50-85% byweight, Cr: 15-50% by weight and Al: 1-6% by weight.
 13. The combinationof a diesel engine and particulate trap as claimed in claim 3, whereinsaid heat-resistant metal is an Ni-based alloy containing Ni: 50-85% byweight, Cr: 15-50% by weight and Al: 1-6% by weight.
 14. The combinationof a diesel engine and particulate trap as claimed in claim 4, whereinsaid heat-resistant metal is an Ni-based alloy containing Ni: 50-85% byweight, Cr: 15-50% by weight and Al: 1-6% by weight.
 15. The combinationof a diesel engine and particulate trap as claimed in claim 5, whereinsaid heat-resistant metal is an Ni-based alloy containing Ni: 50-85% byweight, Cr: 15-50% by weight and Al: 1-6% by weight.
 16. The combinationof a diesel engine and particulate trap as claimed in claim 1, whereinsaid trap also includes an electric heater disposed in said case to burnparticulates collected by said at least one filter element.
 17. Thecombination of a diesel engine and particulate trap as claimed in claim2, wherein said trap also includes an electric heater disposed in saidcase to burn particulates collected by said at least one filter element.18. The combination of a diesel engine and particulate trap as claimedin claim 3, wherein said trap also includes an electric heater disposedin said case to burn particulates collected by said filter elements. 19.The combination of a diesel engine and particulate trap as claimed inclaim 4, wherein said trap also includes an electric heater disposed insaid case to burn particulates collected by said filter elements. 20.The combination of a diesel engine and particulate trap as claimed inclaim 5, wherein said trap also includes an electric heater disposed insaid case to burn particulates collected by said at least one filterelement.
 21. The combination of a diesel engine and particulate trap asclaimed in claim 6, wherein said trap also includes an electric heaterdisposed in said case to burn particulates collected by said at leastone filter element.
 22. The combination of a diesel engine andparticulate trap as claimed in claim 11, wherein said trap also includesan electric heater disposed in said case to burn particulates collectedby said at least one filter element.